An amplifier is an electronic circuit that can increase the power of a received input signal (e.g., a time-varying voltage or current). Each amplifier uses electric power from a power supply to increase the amplitude of a relatively weak input signal in order to generate a relatively strong (amplified) output signal, where the amount of amplification determines the amplifier's gain (i.e., the ratio of output signal voltage, current, or power to input signal voltage, current, or power). Amplifiers can either be separate stand-alone devices, or electrical circuits implemented as part of an integrated semiconductor device.
The quality of an amplifier circuit is often determined by measuring the signal-to-noise ratio of the amplifier circuit's output signal. Along with the desired amplified input signal, each amplifier circuit also outputs noise, which is an unwanted disturbance in the (electrical) output signal that may preclude accurate detection of the desired input signal by downstream circuitry. Noise generated by electronic devices varies greatly as it is produced by several different effects. In communication systems, noise can produce errors or undesired random disturbances of useful information (i.e., the input signal). Signal-to-noise ratio (abbreviated SNR or S/N) is a measure that compares the level of the desired amplified input signal to the level of noise that is transmitted with the amplified input signal. S/N ratio is defined as the ratio of signal power to the noise power, and is typically expressed in decibels (dB), which is calculated using a ratio of the root mean square signal power to the root means square noise power at the amplifier output, and then multiplying the log (base 10) of the ratio value by twenty. Circuit designers strive to minimize a circuit's S/N ratio by way of considering various trade-offs that include impedance matching, choosing a suitable amplifier technology (such as low-noise components), and selecting low-noise biasing conditions.
A low-noise amplifier (LNA) is an electronic amplifier circuit that is often utilized in various integrated circuits to amplify extremely weak and uncertain signal (e.g., received from an antenna), often on the order of microvolts or under −100 dBm (dB referenced vs. 1 mW of power), and amplify it to a more useful level (e.g., about one-half to one volt). LNAs are used in communication devices (e.g., a radio communications system or a cellular telephone), medical instruments and electronic equipment in which the weak input signals that are just above the noise floor. Ideally, LNAs function to capture and amplify very low-power, low-voltage input signals that are within a bandwidth of interest, and to filter out all random background noise that may be received with the input signals. In practice, the detection and removal of all background noise under these conditions, which is known in the art as the unknown signal/unknown noise challenge, is the most difficult of all signal-processing challenges, and current LNA technologies are only able to reduce (i.e., not fully eliminate) background noise generated in the output signal.
LNAs are typically compared based on three primary parameters: noise figure, gain, and linearity (power consumption and efficiency are typically not primary concerns). The noise figure (NF) of an LNA is a measure of degradation of the S/N ratio caused by thermal (and other) effects on components in a radio-frequency (RF) signal chain, where most LNA typically have NF values in the 0.5 to 1.5 dB range. Typical gain is between 10 and 20 dB for a single stage LNA. While providing gain itself is not a major challenge with modern electronics, it is severely compromised by any noise that the LNA may add to the weak input signal, which can overwhelm any benefits of the amplification that the LNA adds. Linearity refers to the relationship between input signal strength and output signal strength, where an amplifier's linearity is determined by how closely the output signal strength varies in direct proportion to the input signal strength. Nonlinearity in an LNA is caused by the resultant harmonics and intermodulation distortion that can corrupt the received input signal, and make demodulation and decoding with a sufficiently low error rate more difficult. Linearity is usually characterized by the third-order intercept point (IIP3), which relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal; the higher the IIP3 value, the more linear the amplifier performance.
Various circuit design level techniques have been used in an attempt to produce LNAs exhibiting improved linearization (i.e., increased IIP3), including inductive source degeneration and multi-gated-transistor linearization. However, there are several disadvantages to addressing LNA linearization using these conventional circuit design level approaches. First, these approaches require additional biasing circuits and devices, which increase the complicacy of the LNA circuit and overall chip size, thereby increasing manufacturing costs by reducing production yields. In addition, these approaches increase the total current consumed by the LNA during operation, and reduce the LNA's power gain. Moreover, these approaches increase RF noise, which reduces the LNA's NF value.
What is needed is an amplifier circuit that achieves improved linearity and avoids the disadvantages of conventional linearization approaches. In particular, what is needed is an amplifier circuit that achieves significant IIP3 improvement without a significant reduction in transducer power gain.